The present invention relates to optical particle detection and, in particular, to a particle detection system with increased sensitivity in the detection of submicron diameter particles.
Contamination control, including particle monitoring, plays a critical role in the manufacturing processes of several industries. These industries require clean rooms or clean zones with active air filtration and require the supply of clean raw materials such as process gases, deionized water, chemicals, and substrates. In the pharmaceutical industry, the Food and Drug Administration requires particle monitoring because of the correlation between detected particles in an aseptic environment and viable particles that contaminate the product being produced. Semiconductor fabrication companies require particle monitoring as an active part of quality control. As integrated circuits become more compact, line widths decrease, thus reducing the size of particles that can cause quality problems. Accordingly, it is important to detect and accurately measure submicron particles of ever-decreasing sizes and numbers per volumetric unit.
To perform particle monitoring, currently commercially available submicron particle detection systems use optical detection techniques to determine the presence, size, and number of particles in a volumetric unit. The foundation of this technology is optical scattering of a light beam and detection of the optical signal after it has been scattered by a sample particle. The standard particle detection approach, which was developed during the late 1980s, entails intersecting, in a region referred to as a xe2x80x9cview volume,xe2x80x9d a light beam and a sample stream containing sample particles. Light scattered from the view volume is collected with optics and focused onto a detection system that collects the volume of light and projects it onto one or more detector elements. The ability of a particle detection system to detect small particles depends on its ability to distinguish between noise and pulse output signals generated from light scattered by submicron sample particles.
What is needed, therefore, is a particle detection system having high submicron particle detection sensitivity.
An object of the invention is, therefore, to provide a particle detection method and system characterized by increased submicron particle detection sensitivity and accurate particle size determination.
The particle detection system of the present invention includes a flow chamber within which a light beam and a fluid stream containing sample particles intersect to form a view volume. The incidence of a sample particle on the light beam causes portions of the light beam to scatter from the view volume in the form of first and second correlated scattered light components. The first correlated scattered light component exits the view volume in a first direction, is collected and focused by a light collection lens system, and is detected by a first detector element of a pair of detector elements located in an array of detector elements. The second correlated scattered light component exits the view volume in a second direction and is incident upon a light reflector. The light reflector reflects in an inverted state the second correlated scattered light component and focuses it into the view volume. The second correlated scattered light component then passes through the view volume, is collected and focused by the light collection lens system, and is detected by a second detector element of the pair of detector elements.
Each detector element in the pair of detector elements detects the incidence of light and generates a pulse output signal, the magnitude of which depends on the intensity of the incident scattered light component. A signal processing system performs analog or digital signal processing of only those pulse output signals that are temporally and spatially coincident such that both of the first and second detector elements of the pair of detector elements concurrently generate pulse output signals. If each of the pulse output signals concurrently crosses its associated threshold, the signal processing system filters the pulse output signals to remove noise and amplifies the signals to generate a final pulse output signal indicating the presence and size of the sample particle.
Signal enhancement results from the required temporal and spatial coincidence of pulse output signals corresponding to the same sample particle. Because sample particles are counted only when both detector elements of a pair concurrently detect a scattered light component and when the resultant pulse output signals exceed a predetermined threshold, randomly occurring noise pulses or excursions are unlikely to concurrently contact both of the detector elements in the pair. Specifically, the probability that two pulse output signals will concurrently exceed the predetermined threshold is equal to the square of the probability that an individual pulse output signal will exceed the threshold. The coincidence function permits the use of a lower threshold for a given false count rate because most noise is random and will not concurrently trigger both detector elements in the pair. Use of a lower threshold facilitates the detection of smaller sample particles.
The particle detection system also preferably includes a noise detection and cancellation system that prevents noise from triggering a coincidence event. The preferred noise detection and cancellation system includes a noise detector that is positioned to monitor only the laser beam and a cancellation unit that removes (i.e., by subtraction or division) the signal generated by the noise detector from the signal generated by each detector element.
The particle detection system of the present invention has an increased ability to distinguish between noise and low-amplitude pulse output signals caused by small diameter particles. The required temporal and spatial coincidence of pulse output signals results in signal enhancement. Because sample particles are counted only when both symmetrically opposed detector elements concurrently detect scattered light correlated components having a pulse output signal that exceeds a predetermined threshold, the incidence of randomly occurring noise pulses or excursions causing a false signal is significantly decreased. Specifically, the probability that two pulse output signals will concurrently exceed the predetermined threshold is equal to the square of the probability that an individual pulse output signal will exceed the threshold. Consequently the threshold for a given false count rate may be lowered by more than a factor of the square root of two while maintaining the desired overall false count rate. Thus the coincidence function allows the use of a lower threshold setting without increasing the incidence of false particle signals, since most noise is random and is unlikely to concurrently trigger both detector elements of a symmetrically opposed pair. The use of a lower threshold facilitates more accurate detection of smaller diameter particles.
Additional objects and advantages of this invention will be apparent from the following detailed description of preferred embodiments thereof, which proceeds with reference to the accompanying drawings.